An electric power steering apparatus (EPS) which provides a steering system of a vehicle with an assist torque by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the calculated current command value.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a torque control section 31 to calculate a current command value Iref1, and the calculated current command value Iref1 is inputted into a subtracting section 32B, where a detected motor current value Im is subtracted from the current command value Iref1. A deviation I (=Iref1−Im) which is the subtracted result in the subtracting section 32B is controlled in the current control section 35 such as a proportional-integral (PI) control and so on. The voltage control value Vref obtained by the current control is inputted into a PWM-control section 36 which calculates duty command values, and PWM-drives the motor 20 through an inverter circuit 37 by means of a PWM signal. The motor current value Im of the motor 20 is detected by a motor current detector 38, and is inputted and fed back to the subtracting section 32B. Further, a rotational angle sensor 21 such as a resolver is connected to the motor 20 and a steering angle θ is detected and outputted.
In such the electric power steering apparatus, when a large assist torque from the motor is applied to the steering system near the maximum steering angle (the rack end) thereof, a strong impact (a shock) occurs at a time when the steering system reaches at the maximum steering angle, and the driver may feel uncomfortable because of generating the hitting noise (noisy sound) due to the shock.
The electric power steering apparatus that includes a steering angle judging means for judging whether the steering angle of the steering system reaches at a front by a predetermined value from the maximum steering angle and a correcting means for correcting which decreases the assist torque by reducing the power supplied to the motor when the steering angle reaches at a front by a predetermined value from the maximum steering angle, is disclosed in Japanese Examined Patent Application Publication No. H6-4417 B2 (Patent Document 1).
Further, the electric power steering apparatus disclosed in Japanese Patent No. 4115156 B2 (Patent Document 2) is that: the electric power steering apparatus that judges whether an adjustment mechanism becomes near an end position or not, controls a driving means so as to decrease a steering assist when the adjustment mechanism reaches at near the end position, and evaluates an adjustment speed determined by a position sensor in order to determine the speed when the adjustment mechanism approaches to the end position.